1. Field Of The Invention
This invention relates to a lead chuck constituting a lead feeding mechanism of a mechanical pencil, and more particularly a lead chuck of a mechanical pencil in which a relative sliding between a lead holding part of the lead chuck and an outer circumference of a lead is prevented, a biting action against the lead is positively performed, and even if an outer circumference of the lead is scraped by the lead holding part, the biting against the lead is positively performed and at the same time a clogging caused by lead dust or the like is prevented.
2. Description Of The Prior Art
In general, a lead chuck of a mechanical pencil is installed between a lead pipe arranged at a rear end of a shaft and a slider arranged at an extremity end of the shaft. The lead chuck is constructed such that it is equally divided into three portions by axial slots formed at the extremity end. These chuck pieces are merely formed by three repetitive cuts or by injection molding a complex mold so as to make a lead chuck.
The above-mentioned lead chuck with a three-way divided end shows a reduction in its productivity and has a substantial start-up cost due to the fact that its manufacturing method is complicated as described above. Since the mere forming of a three-way divided lead holding part with slots fails to provide sufficient lead holding force, it is sometimes found that a toothed part having some lead biting teeth is required. However, forming of such teeth causes a complex shape and structure of the lead chuck in addition to the above-mentioned structure. Thus, in such a case, after the slot forming operation is carried out, the teeth must be formed by a tapping operation or the like, resulting in a substantially deteriorated efficiency of manufacture.
In view of the foregoing, it is proposed by the present applicant to provide a lead chuck of a two-way divided fully separate structure in place of the above-mentioned lead chuck of three-way divided structure with a slotted extremity end.
The lead chuck of this two-way divided fully separate type is constructed such that the two chuck members are formed as a small cylindrical cylinder axially divided into two separate portions which have approximately similar shapes and are integrally assembled in a way such that their divided surfaces contact each other so as to hold a lead therein. In order to hold the lead, the divided surfaces of the above-mentioned chuck members are formed with a lead insertion hole along their axial directions at a substantially central part, and at the extremity ends of these chuck members are formed small diameter lead holding holes. Each of the outer circumferences of the chuck members is provided with a ball holding part to secure balls therein as they are guided by an inner circumferential tapered surface of the chuck fastening member at the extremity end to fasten and hold the chuck member. At the rear end, each of the resilient means engaging flanges is formed in a semi-circumferential shape for resiliently biasing against the sleeve. At the divided surfaces, positioned to correspond to the semi-circumferential flanges, are formed some supporting points for use in expanding or retracting each of the chuck members of the lead chucks in a forward or rearward direction. Further, at each of the rear end portions of the chuck members is formed an outer circumferential tapered surface for use in expanding an extremity end of the chuck member when the rear end of the chuck member is pushed out in a rearward direction by knocking against a knocking part which is projected and arranged at an axial rear end of the mechanical pencil.
By use of a two-way divided fully separate type lead chuck constructed as described above, it is possible to eliminate disadvantages such as a reduction in productivity or an increased cost of manufacturing or the like, during formation of the lead holding teeth even when the chuck member is to be formed by injection molding or the like. Use of this type of chuck assures ease of manufacture and that the lead holding force of the lead holding teeth will be sufficient.
However, even if the above-mentioned conventional type of lead chuck of a mechanical pencil is used, a so-called lead diameter reducing phenomenon occurs in which teeth formed at the lead holding part on each of the chuck portions cuts into an outer circumference of the lead as the mechanical pencil is used in. This occurs even though the intent is to provide a positive holding of the lead by formation of the teeth, and thus the lead holding force is gradually decreased and sliding of the lead may occur.
The above-mentioned lead chuck is for use in a mechanical pencil of a rear end knock type. A mechanical pencil of the automatic knock type has means by which a pressing of the extremity end of the lead against the surface of a writing paper causes the lead to be automatically fed when the writing pressure is released during a writing operation. A mechanical pencil of extremity end knock type used together with this automatic lead feeding feature or by itself has had the problem of a lack of projection amount of the lead. An outer circumference of the lead is cut by the teeth of the lead chuck divided surfaces, thus causing the lead to be further reduced in its diameter which, in turn, causes a reduction in holding force. Different amounts of holding force are required during the pressing of the pencil against the writing sheet and during its released state.
This lead diameter reduction phenomenon causes lead dust to be accumulated in a clearance of the lead insertion hole of the lead chuck or in another clearance between the lead chuck and the sleeve or the like and results in various lead feeding operations such as a rear end knock, automatic lead feed and extremity end knock or the like to be delayed due to clogging of the lead dust. The operating characteristics are deteriorated and in the worst state, the lead feeding function is not operable at all.